A method for studying biological cells with the aid of stimulating them with sinusoidal alternating current and studying the non-linear response of these cells has been described in the patent application WO 92/04630. The above-mentioned cells are located in the studied analytical environment, for example in the form of a suspension or biological tissue.
The method described in the aforementioned patent application is used to study the properties of the solution or the biological tissue as a whole. In the case of in vivo studies, the existence of complex biological tissue systems which differ significantly as far as biophysical and biochemical parameters are concerned creates the main problem. In in vivo studies, the whole organs will be investigated in the majority of cases rather than tissues or groups of identical cells.
Thus, the method described in the aforementioned patent application could not be used to diagnose secondary caries because a tooth is an organ composed of many tissues whose properties differ significantly, including hard tissues which have unique physical and chemical properties decidedly different from the properties of cell solutions or soft tissues which were supposed to be studied by the method described in the prior art invention.
When studying a tooth using a method from the aforementioned patent application, it is impossible to mechanically separate one selected tissue without destruction.
A tooth is composed of mineral components, electrolytes and biological cells, also those located in soft tissues.
Thus, both the bio-impedance study method (including harmonic response study) of a human tooth in vitro or in vivo as a whole as well as the analysis of the results differ fundamentally from those of the isolated biological tissues.
Electrical properties (conductivity and dielectric permittivity) of a tooth or hard tissue in general are fundamentally different than the ones of soft tissue.
There are other sources of non-linearity in the tooth tissue than the ones described in the aforementioned patent application.
At the moment secondary caries diagnosis is carried out using good eye and bitewing X-rays.
When using the above-mentioned method a tooth should be cleaned from the residue and dried. It also exposes the patient to potentially harmful ionizing radiation. The method is time-consuming and does not offer immediate diagnosis.
There are also modern optical methods, e.g. caries detection device Diagnodent which have a limited scope of detection. Amalgam fillings are not transparent and the performance of the device demonstrates low sensitivity and specificity.
The other method which has a wide application in the enamel quality diagnosis are bio-impedance measurements. They are popular in studying the root canal fillings, and lately have been used to study enamel quality among other things.
Bio-impedance measurements of enamel consist in the analysis of its ac impedance spectrum and allow to determine precisely the enamel dielectric and conductivity parameters which change significantly when the disease processes are present.
This method is especially accurate when the structure of a tooth is known. Moreover, the examination is non-invasive, it does not take much time and the cost of producing a measurement apparatus is relatively low. These advantages have already been used in the commercially available devices for bio-impedance enamel analysis (e.g., www.cariescan.com).
The above-mentioned method demonstrates very high sensitivity [correct identification of carious sites] in a physical rather than medical sense but very low resolution [correct identification of sound sites] which signifies that the response can be influenced by a number of biophysical factors and phenomena such as for example water presence, surface cleanliness including microbiological one. It cannot be used to study secondary caries.
Thus, there is still a need for a method and a device for secondary caries diagnosis which is quick, does not expose patients to ionizing radiation, has high resolution allowing to detect water environment presence such as bacterial microflora or changes in sound enamel caused by secondary caries development. This method should also consider specific electrical properties (conductivity and dielectric permittivity) of a tooth and its filling (the fact that amalgam is not transparent).